Increasing environmental awareness and respect for valuable natural resources has created an abiding concern in the area of underground leak detection. This is especially true with regard to fuel tank storage and dispensing systems. The industry has been continuously in search of new technologies and enhancements to existing technologies for underground leak detection, particularly pipeline leak detection.
Conventional leak detection methods for pipelines involve pressurizing the line and then observing a pressure decay when there is a leak. This method is subject to false readings due to temperature changes of the pressurized liquid caused by heating or cooling from the environment. A small temperature change, specifically a reduction in temperature, can cause a large change in pressure of liquid, which is, for most purposes, incompressible. In pipelines, where a relatively few number of gallons of liquid may be involved, a very small amount of liquid volume, that is, flow, causes a large change in pressure. For example, a typical gasoline station may have 120 feet of pipe connected to one storage tank. There may be about 20 gallons of fuel in that length of pipe. A one-degree F reduction in liquid temperature at a typical temperature coefficient of expansion results in a detectable reduction in pressure. However, that one degree of temperature reduction results in a volume change of only 0.014 gallons of liquid.
Attempts to use currently available flow meters or flow sensors have been satisfactory for some purposes but they are insufficiently accurate to detect fluid flow at the minimum level required by the Environmental Protection Agency with respect to fuel storage and dispensing systems. This minimum level is currently 0.1 gallon per hour and any leak detection system for either fuel tanks or piping which does not achieve this minimum detection level will not be acceptable.
The apparent leak rate of 0.014 gallons per hour, if the one degree F reduction took place over the course of one hour, would be undetectable by a conventional flow meter. By way of further example, a leak rate of 0.1 gallon per hour amounts to only two drops per second. That is below the threshold leak rate detectable by currently available flow meters which themselves typically leak at more than two drops per second.
The description of some of the prior art pipeline leak detectors is set forth in a paper entitled "Service Station Pipeline Leak Detectors," by N.D. Wolff, presented at the Underground Storage Tank Leak Prevention Seminars conducted by the California State Water Resources Control Board in July and Aug. 1988. This describes some of the pressure related leak detectors and some of the alternative leak detector devices.